The present invention relates to a multi-stage turbocharger system. Particularly, but not exclusively, the present invention relates to a two-stage turbocharger system and a method for operating the same.
Turbochargers are well known devices for supplying air to the intake of an internal combustion engine at pressures above atmospheric pressure (boost pressures). A conventional turbocharger essentially comprises an exhaust gas driven turbine wheel mounted on a rotatable shaft within a turbine housing connected downstream of an engine outlet manifold. Rotation of the turbine wheel rotates a compressor wheel mounted on the other end of the shaft within a compressor housing. The compressor wheel delivers compressed air to the engine intake manifold. The turbocharger shaft is conventionally supported by journal and thrust bearings, including appropriate lubricating systems, located within a central bearing housing connected between the turbine and compressor wheel housings.
In known turbochargers, the turbine stage comprises a turbine chamber within which the turbine wheel is mounted; an annular inlet passageway defined between facing radial walls arranged around the turbine chamber; an inlet arranged around the inlet passageway; and an outlet passageway extending from the turbine chamber. The passageways and chambers communicate such that pressurised exhaust gas admitted to the inlet chamber flows through the inlet passageway to the outlet passageway via the turbine and rotates the turbine wheel. It is also known to improve turbine performance by providing vanes, referred to as nozzle vanes, in the inlet passageway so as to deflect gas flowing through the inlet passageway towards the direction of rotation of the turbine wheel.
Another known approach to improving turbocharging efficiency for an engine with a wide speed/load range is to provide a sequential two stage turbocharging system, comprising one relatively small high pressure (HP) turbocharger and another relatively large low pressure (LP) turbocharger. The turbochargers are arranged in series so that exhaust from the engine flows first through the smaller turbine of the HP turbocharger and then through the larger turbine of the LP turbocharger. A valve-controlled bypass path is provided for allowing exhaust gas to bypass the HP turbine for instance at high engine speeds and/or loads. Similarly, the compressors of the two turbochargers are also arranged in series, with air flowing first through the relatively large compressor of the LP turbocharger and then through the relatively small compressor of the HP turbocharger. Again, a valve controlled bypass is provided to allow the inlet air to bypass the compressor of the HP turbocharger for instance at high engine speeds and/or loads.
In a sequential two-stage turbocharging system disclosed in WO 2008/015400, exhaust gas flow through the HP turbine, LP turbine and HP bypass path is controlled by a single rotary valve which may be positioned upstream or downstream of the HP turbine. In more detail, a primary exhaust gas flow path comprises an inlet portion which delivers exhaust gas to the HP turbine and an intermediate portion which delivers exhaust gas from the HP turbine to the LP turbine. The bypass path communicates between the inlet portion of the primary flow path and the intermediate portion of the primary flow path. The exhaust gas flow control valve is a rotary valve located at a junction of the bypass path and the primary path (which may be the inlet portion of the primary path if the valve is upstream of the HP turbine, or the intermediate portion of the primary path if the valve is downstream of the HP turbine). The rotary valve comprises a valve rotator rotating in a valve chamber and operable to permit or block flow through the bypass path and/or the primary path that is again either the inlet or intermediate portions of the primary path depending on whether the valve is upstream or downstream of the HP turbine respectively. In one embodiment in which the valve is upstream of the HP turbine, the valve comprises an inlet port communicating with the inlet portion of the primary flow path, a primary outlet port communicating with the inlet portion of the primary flow path (so that flow to the HP turbine flows through the valve via the inlet port and primary outlet port), and a bypass port which provides an outlet to the bypass path which communicates with the inlet of the LP turbine (which may be via the intermediate portion of the primary flow path). In another embodiment in which the valve is downstream of the HP turbine, the valve comprises a primary inlet port communicating with the intermediate portion of the primary flow path, and a primary outlet port communicating with the LP turbine inlets (via the intermediate portion of the primary flow path), and a bypass inlet port communicating with the by pass path.
By rotation of the valve rotor to open, close or partially open/close individual ports the turbocharger system may be operated in various modes including a normal fired mode for modulating the exhaust gas flow to the HP and LP turbines, a braking mode (non-fired mode) in which the valve is operated to provide a braking back pressure in the exhaust system, and an exhaust gas heating mode (fired operating mode) in which the valve is operated to restrict exhaust gas flow to thereby heat the exhaust gas flow.
Embodiments of the sequential turbocharger system of WO 2008/015400 may include a wastegated LP turbine. The skilled person will appreciate that a wastegate is a valve controlled bypass flow path around the turbine.